The present invention relates to the purification of gases, and more particularly to a method of purifying flue gases which contain noxious gases such as SO3.
SO3 is a noxious gas that is produced from the combustion of sulfur-containing fuel. When present in flue gas, the SO3 can form an acid mist that condenses in electrostatic precipitators, ducts or bag houses, causing corrosion. SO3 at concentrations as low as 5-10 ppm in exhaust gas can also result in white, blue, purple, or black plumes from the cooling of the hot stack gas in the cooler air in the atmosphere.
The effort to reduce NOx emissions from coal-fired power plants via selective catalytic reactors (SCRs) has resulted in the unintended consequence of oxidizing SO2 to SO3 and thereby increasing total SO3 emissions. SCRs employ a catalyst (typically vanadium pentoxide) to convert NOx to N2 and H2O with the addition of NH3, but there is also an unintended oxidation of the SO2 to SO3. Although the higher stack SO3 concentrations are still relatively low, the emissions can sometimes produce a highly visible secondary plume, which, although unregulated, is nonetheless perceived by many to be problematic. Efforts to reduce the SO3 levels to a point where no secondary SO3 plume is visible can impede particulate collection for stations that employ electrostatic precipitators (ESPs). SO3 in the flue gas absorbs onto the fly ash particles and lowers fly ash resistivity, thereby enabling the ESP to capture the particle by electrostatic means. Some plants actually inject SO3 to lower fly ash resistivity when ash resistivity is too high.
SO3 reacts with water vapor in the flue gas ducts of the coal power plant and forms vaporous H2SO4. A portion of this condenses out in the air heater baskets. Another portion of the sulfuric acid vapor can condense in the duct if the duct temperature is too low, thereby corroding the duct. The remaining acid vapor condenses either when the plume is quenched when it contacts the relatively cold atmosphere or when wet scrubbers are employed for flue gas desulfurization (FGD), in the scrubber's quench zone. The rapid quenching of the acid vapor in the FGD tower results in a fine acid mist. The droplets are often too fine to be absorbed in the FGD tower or to be captured in the mist eliminator. Thus, there is only limited SO3 removal by the FGD towers. If the sulfuric acid levels emitted from the stack are high enough, a secondary plume appears.
Dry sorbent injection (DSI) has been used with a variety of sorbents to remove SO3 and other gases from flue gas. However, DSI has typically been done in the past at temperatures lower than around 370° F. because equipment material, such as baghouse media, cannot withstand higher temperatures. Additionally, many sorbent materials sinter or melt at temperatures greater than around 400° F., which makes them less effective at removing gases. The reaction products of many sorbent materials also adhere to equipment and ducts, which requires frequent cleaning of the process equipment.